Commentaire invité par David Archer (Univ. Chicago) (traduit par Thibault de Garidel)
La notion que la durée de vie du CO2 émis dans l’atmosphère par l’action humaine se situe entre quelques décennies � quelques siècles, est omniprésente dans les revues de vulgarisation scientifique ainsi que celles spécialisées. La réalité est que chaque litre de pétrole que nous utilisons en faisant le plein de notre voiture continuera � affecter le climat pendant les prochaines dizaines et mêmes centaines de milliers d’années.
L’agence américaine de protection de l’environnement (EPA) estime par exemple la durée de vie du CO2 entre 5 et 200 ans, dans son inventaire des sources et puits de gaz a effet de serre aux USA [1]. J’ai également vu des “centaines d’années” dans des articles scientifiques et dans la littérature “environnementaliste”. David Goodstein dans son excellent livre “The End of the Age of Oil” dit : ” si nous nous arrêtions brusquement de consommer du carbone fossile, le cycle naturel du carbone restaurerait la concentration initiale en environ un millier d’années”. J’imagine que Goodstein applique conservativement quelques siècles multipliés par un facteur donné pour dériver son millier d’années, mais il fait l’hypothèse implicite que la concentration atmosphérique en CO2 reviendra a son niveau de 1750. Le problème c’est que ce n’est pas le cas.
Quand vous émettez une dose de CO2 “neuf” [NdT: dans le sens nouveau dans le cycle du Carbone ; ce carbone “neuf” est en réalité du carbone très ancien, fossilisé sous forme de pétrole ou de gaz] dans l’atmosphère, environ les trois quarts sont dissous dans l’océan et disparaissent, en fonction de la quantité émise. Le quart restant reste � être neutralisé en réagissant avec les roches ignées ou carbonatées sur terre et dans les océans. Les réactions avec ces roches restaurent également le pH de l’océan du pic acide lié au CO2. Mon modèle indique qu’environ 7% du carbone émis de nos jours, y sera toujours dans 100 000 ans [7]. Mon estimation de la durée de vie, � partir de tous les processus en jeu, est d’environ 30 000 ans. C’est un chiffre décevant car il est très influencé par la lenteur de ces réactions qui forment une “queue”. Néanmoins, s’il faut choisir un chiffre afin de simplifier les discussions avec le grand public, alors le choix de quelques centaines d’années est raisonnable car il prend en compte les trois quarts de l’histoire, et la partie qui nous affecte nous et nos durées de vie.
Néanmoins, il est exagéré de ne pas prendre en compte cette “queue” sous le prétexte de la vulgarisation. Les principales calottes de glaces, en particulier au Groenland [8], les dépôts de clathrate de méthane [9], et l’évolution future des cycles glaciaires/interglaciaires [10] peuvent être affectés par cette longue “queue”. Une image plus juste pour le grand public est donc que le CO2 émis reste environ quelques centaines d’années, alors que 25% reste indéfiniment.
L’idée que cette durée de résidence est extrêmement longue n’est pas nouvelle, et notre compréhension n’a pas beaucoup changé depuis que cet effet a été pour la première fois prédit en 1992 [2]. Il est possible d’estimer la magnitude de cet effet en utilisant simplement la thermodynamique du CO2. Il peut être également avancé que comme nous ne connaissons pas pourquoi le CO2 était moins élevé pendant la dernière période glaciaire, nous ferions mieux de ne pas faire de pronostics pour le futur. Ce � quoi je répondrais, que le CO2 semble jouer le rôle d’un amplificateur des changements climatiques forcés par les paramètres orbitaux, et qu’en tout état de cause, nous devons nous attendre � ce que le cycle du carbone amplifiera encore plus notre propre forçage climatique, plutôt que l’atténuer. Si le passé peut nous servir de guide, � long terme, les surprises du CO2 � l’avenir ne risquent pas de nous aider beaucoup.
Une longue durée de vie pour le CO2 est également compatible avec un événement isotopique enregistré dans les sédiments océaniques il y a environ 55 millions d’années, l”Optimum thermique Paléocene/Eocene” – (Paleocene/Eocene Thermal Maximum). Ces sédiments révèlent qu’un relargage abrupt de carbone dont les isotopes les plus légers étaient abondants, a déclenché un rechauffement rapide des masses d’eau océaniques profondes d’environ 5 degrés Celsius. Il a fallu attendre environ 100 000 ans pour que le signal isotopique du carbone et la température (déduite des isotopes de l’oxygène) reviennent � des concentrations similaires � celles initiales. Même si le carbone relargué était principalement sous forme de méthane, il aurait du s’oxyder sous forme de CO2 en quelques décennies, mais apparemment, il est resté comme le CO2 suffisamment longuement pour permettre de réchauffer les eaux profondes avant qu’il ne s’en aille.
Les durées de vie les plus courtes, telles que l’estimation de 5 ans faite par l’EPA, est déduite du flux d’échange de CO2 entre atmosphère et océan, qui est d’environ 200 Gt C/an (1 Gigatonne de Carbone correspond a 1012kg de carbone) dans chaque direction. Comme les échanges entre océan et atmosphère fonctionnent � double-sens, ceci n’a pas d’implications sur l’absorption par l’océan du”nouveau” CO2 [NdT: anthropogénique], qui dépend du déséquilibre entre les flux entrant et sortant. Ce déséquilibre n’est que de 2Gt C/an.
Même les flux nets modernes ont tendance � sous-estimer la durée de vie réelle du réchauffement global. L’atmosphère contient environ 160 Gt C qu’il ne contenait auparavant [NdT: avant l’ère industrielle, ~1850 ap JC]. Si on divise simplement ce nombre par le flux entrant de CO2 dans l’océan de 2Gt C/an, on obtient une durée d’assimilation d’environ 80 ans. Ce résultat est environ 4 fois plus court que les échelles de temps d’équilibrage entre océan et atmosphère modélisées. Je pense que le problème résulte de la simplicité du calcul. La concentration atmosphérique en CO2 ne cesse d’augmenter, et envahie également l’océan comme dans les régions chaudes de l’océan. Si la concentration atmosphérique en CO2 cessait d’augmenter, les eaux chaudes de l’océan seraient saturées en une année ou deux, le taux de pénétration du CO2 dans l’océan decroitrait en retour, et par conséquent l’estimation de la durée de vie de carbone par cette méthode augmenterait. Les différentes régions de l’océan atteignent un équilibre avec l’atmosphère � différentes échelles de temps, allant de l’année pour les masses d’eaux de surface de l’océan tropical au millier d’années pour les masses d’eaux de l’océan profond. Généralement, les simulations numériques démontrent que le temps de mise a l’équilibre du CO2 est d’environ de quelques siècles [5, 6, 11, 12]. Un problème supplémentaire pour ces deux estimations différentes est qu’elles nous indiquent implicitement que la concentration en CO2 va revenir � sa concentration initiale, ce qui n’est pas le cas.
L’utilisation d’un calcul permettant de comparer les conséquences des différents gaz � effet de serre, nommées le potentiel de réchauffement global (GWP), permet également d’estimer la durée de vie du carbone, estimations biaisées vers des valeurs basses. Certains gaz ont un effet par molécule, beaucoup plus fort sur le bilan thermique global ; le méthane, par exemple, a un potentiel beaucoup plus important que le CO
Il est également possible, que nous-mêmes dont l’espérance de vie est de 77.2 ans [aux USA] ne voulons pas nous soucier des impacts climatiques de notre consommation de carbone fossile dans 100 000 ou même 1000 ans. C’est une posture parfaitement rationnelle, pour laquelle je n’ai pas d’opposition. Les négociations sur l’évolution du climat sont basées sur des projections et scénarios du GIEC � l’horizon 2100, bien avant l’an 100 000. Mais même 2100 apparaît comme étant inimaginablement distant en regard de la vitesse des changements sociaux et technologiques modernes. D’un autre coté, les déchets nucléaires persistent pour des millions d’années pour certains radioisotopes comme l’iode 129. Le public semble trouver cette information pertinente, ainsi la véritable longévité des changements climatiques dus � l’action humaine peut également être considéré comme tout aussi pertinente pour prendre des décisions immédiates. Dans tous les cas, les faits reportés doivent être justes, plutôt qu’issus du préjugé que personne ne se soucie d’impacts climatiques � venir qui dureront des milliers d’années et plus.
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Références :
- (with the subscript “No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes”) (EPA (2005), Inventory of U.S. Greenhouse Gas Emissions and Sinks Draft Report: 1990 -2003, U.S. Environmental Protection Agency, Office of Atmospheric Programs
- Walker, J.C.G. and J.F. Kasting, Effects of fuel and forest conservation on future
levels of atmospheric carbon dioxide. Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 1992. 97, 151-189. - Joos, F., et al., An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake. Tellus, Ser. B, 1996. 48: p. 397-416.
- Jain, A.K., et al., Distribution of radiocarbon as a test of global carbon cycle models. Global Biogeochem. Cycles, 1995. 9: p. 153-166.
- Archer, D., H. Kheshgi, and E. Maier-Riemer, Multiple timescales for neutralization of fossil fuel CO2. Geophys. Res. Letters, 1997. 24: p. 405-408.
- Archer, D., H. Kheshgi, and E. Maier-Reimer, Dynamics of fossil fuel CO2 neutralization by marine CaCO3. Global Biogeochem. Cycles, 1998. 12: p. 259-276.
- Archer, D., Fate of fossil-fuel CO2 in geologic time. J. Geophys. Res. Oceans, in press.
- Huybrechts, P. and J. De Wolde, The dynamic response of the Greenland and Antarctic ice sheets to multiple-centure climatic warming. J. Climate, 1999. 12: p. 2169-2188.
- Archer, D.E. and B. Buffett, Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing. Geochem., Geophys., Geosys., 2005. 6(3): p. doi: 10.1029/2004GC000854.
- Archer, D. and A. Ganapolski, A movable trigger: Fossil fuel CO2 and the onset of the next glaciation. Geochem., Geophys., Geosys., in press.
- Sarmiento, J.L., U. Siegenthaler, and J.C. Orr, A perturbation simulation of CO2 uptake in an ocean general circulation model. J. Geophys. Res., 1992. 97: p. 3621-3645.
- Sarmiento, J.L. and C.L. Quéré, Oceanic carbon dioxide uptake in a model of century-scale global warming. Science, 1996. 274: p. 1346-1350.
- Jain, A.K., et al., Radiative forcings and global warming potentials of 39 greenhouse gases. J. Geophysical Res., 2000. 105(D16): p. 20,773-20,790.
I know it’s not as well understood — but what is the approximate time frame over which CO2 values returned to normal after the Permian/Triassic mass extinction event and the catastrophic deglaciations in the late Precambrian?
I seem to remember estimates of around 3 million years for the Scythian period.
Some folks at NASA recently discussed the use of a fluorochlorocarbons to “terraform” Mars. I assume the half lives of these compounds are closer to that of methane — and yet the effects are supposed to be ~ permanent. The message I take home is that some folks in the government believe in positive feedbacks and climate instability.
I do care about very long term effects, because my concern regarding GW from the very beginning was how I was impacting others, including those in the future, not what problems I may face personally within my lifetime. I do think this is the concern of most environmentalists, and most people with a developed and mature conscience. That seems to be the concern behind the nuclear waste, as well.
Thanks for the information. It makes each little reduction of GHGs all that more important.
My understanding was that about 60% of the additional carbon we pump into the atmosphere each year is immediately sequestered by natural processes, presumably largely biological. Intuitively this seems very hard to reconcile with the sort of geological timescales discussed above.
A quick read of your paper (reference 7) brought up some questions.
The statement is made that “Atmospheric pCO2 approaches equilibrium on a time scale of ~300yr”. In none of your model scenarios involving realistic near-term Carbon releases (1000 gigatonnes or less, based on the consideration that the 1990-1999 release is estimated in IPCC TAR to be 6.3 gigatonnes per year) does more than 20% of the injected CO2 remain in the atmosphere for 1000 years. It is based on these considerations that reasonable people consider the bulk of the CO2 to have a century scale life cycle in the atmosphere.
Your model seems not to take into account the terrestrial sinks for CO2. When your model neglects the Calcium Carbonate processes and silicate weathering, it predicts that there will be *NO* change in atmospheric CO2 after the ~300y ocean uptake is complete. Is this a reasonable assumption?
While it is a very important point for the lay person to know that the acidification of the ocean by CO2 (it combines with water to produce dilute Carbonic Acid) can reduce the effectiveness of the Calcium Carbonate processes at sequestering Carbon (and can even reverse it, by dissolving Calcium Carbonate), your model chemistry seems quite simplistic. No mention is made of the myriad of other processes (some of which operate on short time scales) whereby the acidity of the ocean is neutralized without involving CaCO3. How robust is your ocean chemistry model?
Your finding of a “long tail” to the remaining 7 to 20% of the CO2 seems to hinge entirely on the ocean-acidity/CaCO3 argument and a neglect of other chemical and terrestrial sequestration processes which operate on short time scales.
Finally I found a few typographical errors in the paper, which I hope your editors will find, e.g. “start contrast” (4th line from bottom, P. 11).
I am trying to critique various proposals for using CO2 uptake in forests to compensate for power-plant emissions of fossil-fuel-derived CO2 and other GHG. Some emerging compliance regulations incorporate the idea that such forests need survive only 100 years to be effective (for these compensation purposes). Archer’s work suggests that the 100-yr rule is an artefact of the 1997 IPCC decision to confine attention to scenarios that end in 2100. Does the 2100 scenario endpoint have some legal basis in the Kyoto Protocol itself?
Archer’s information seems to imply that sequestration “offsets” in forests, soils, and the like are pretty pontless, and that we ought to focus on reducing emissions, period. (Including, of course, the kinds of reductions achievable via changes in land use–changes that probably would include preservation of existing forests, no-till agriculture, and the like.)
I’d appreciate feedback on this interpretation.
I recently ran across this, Dian, while looking up something else:
Martinelli N 2004. Climate from dendrochronology: latest developments and results. Global and Planetary Change 40:1-2 pp. 129-139. [On a global scale, this supposed positive influence on wood production and forest regeneration was thought to have the possibility of balancing the CO2 increase by carbon sequestering through photosynthesis. On the contrary, tree-ring records indicate that, under recent climate warming, drought may have been an important factor in limiting carbon uptake in a large portion of the boreal forest, one of the planet’s major potential carbon sinks. If this limitation in growth due to drought stress is sustained, the future capacity of northern latitudes to sequester carbon may be less than currently expected.]
It’s a conclusion they make as they studied why some researchers are having trouble using recent tree rings from certain spp. and microclimates in some areas.
HTH,
D
Very interesting. Please relate your post to this source The Oceanic Sink for Anthropogenic CO2 (Science, Vol. 305, 16 July 2004). The bottom line is — sorry for the long quote, but this is worth reading — that
In addition, in Table 1 of the referenced source, only about 50% of the estimated emissions from fossil fuels over the nearly 200 year period from 1800 to 1994 (given in petragrams Pg) is taken up by the oceans, disregarding estimated and highly uncertain source emissions from land use. If the low source estimate of these land use changes over that period is taken into account, the percentage taken up by the oceans decreases. Furthermore, the data shown in the same source show that ocean CO2 uptake as a percentage for the period 1980-1999 appears to be decreasing. Since the message I’m getting from the Archer post is that about 75% of atmospheric CO2 will be taken up by the oceans over an approximately 300 year period, given some baseline stabilization of pCO2 levels, I find it hard to reconcile his conclusions on these “short” time scales with the historical data.
Several concepts may clear up some of the uncertainty. Science 4 JULY 2003 VOL 301 “Increase in the Export of Alkalinity from North America’s Largest River” Peter A. Raymond and Jonathan J. Cole offers a strong candidate for the “missing sink” in the form of surprisingly large riverine export of carbon, mostly mineralized. This article challenges the assumption that returning agricultural land to forest is consistently prudent. In some important cases the agricultural use may be sequestering more carbon than forest growth would. Most of this carbon, being mineralized, does not directly interact with the two-way exchange of carbon across the air/ocean surface. It may add as much as ten percent to the total of anthropogenic emissions that wind up in the ocean in the short term, and may join the total pool of mobilized carbon if ocean chemistry is sufficiently disrupted.
In another article, probably in Science in the last three years, but I can’t place the reference at the moment, the rate of permanent fossilization of land-based plant material is defined as about 1% of anthropogenic emissions. This rises to about 2%, if the rate of fossilization of charcoal from forest fires is considered, and both of these rates are supported by the fossil record. Thus land-based plant sequestration should not be regarded as permanent. There is strong evidence that the net movement of carbon in and out of lang-based plants since the beginning of the industrial era has been into the atmosphere.
Carbon sequestration through tree planting and better agricultural soil management practices has a large number of arguments in favor of it. It should be viewed as replacing carbon lost through previous harvest or soil mismanagement, and should NEVER be used as an argument to burn more fossil fuels. At this point we have absolutely zero evidence that any public policies have the capacity to produce a net gain of land based carbon biomass. The only evidence that leans that way, North American and European forest growth, is well within the limits of the known cyclical aspect of the global land-based carbon inventory.
So it looks like about half of our fossil emissions are in the ocean and half are in the atmosphere. I’m basically in agreement with Dr. Archer’s assessment, although I think the greatest complication in his long-term view is whether we saturate the ocean to the point that when surface waters from the fossil era return to the surface some thousand years from now, they become a source, or will they still be a sink. That depends on how much we emit in the next several centuries, and exactly what happens to them in the depths of the ocean. If the answer to the second question is well understood I’d appreciate some direction to the science.
I do also think that the emerging understanding of the effects of acidifying the ocean means that we really ought to try to avoid finding out.
I don’t think humpty dumpty is going to be so easy to put back together again. Consider how fast China is building carbon burning power plants. How likely is it we have seen the peak of carbon emissions?
It sure looks like few people can afford to care about climate impacts thousands of years and more in the future. Most people are too busy going about their lives and will wait until the wolf is at the door. In the next fifty years alone you will see if a lot of people can retire without the world economy falling apart. In a hundred years, the ethnic population of Italy and much of Europe may have shrunk dramatically due to low birth rates.
Unless we have some really compelling economic solution, why wouldn’t the world community permit things to get so bad that the ice caps to melt and the oceans rise?
As a backgrounder it is useful to look at the actual data for the last 45 years. Considering the observed growth of emisions and the observed growth in atmospheric co2 concentration, a sink saturation has not yet been observed, quite opposite the sink has increased from 1000 MtC/y in 1960 to 3000 MtC/y in 2000.
http://hanserren.cwhoutwijk.nl/co2/sink.htm
This is also interesting from the viewpoint of mental models and perceptual frameworks. Browsing the cognitive science journals online, I found a study suggesting that people regularly misapprehend the nature and effects of perturbation: Peter A. White, “Naive analysis of food web dynamics: a study of causal judgement about complex physical systems,” in Cognitive Science; abstract at:
http://www.cognitivesciencesociety.org/abstract/white.html
Another point: during the Cretaceous the level of CO2 in the atmosphere was many times current and temperatures were far higher. But that was the time that the forefathers of the current sea species (mainly planktonic coccoliths) made the white cliffs of Dover and thick carbonate deposits in many places. If (and only if) these species didn’t lose their ability to grow in warmer and more acidic, CO2-rich waters, would that not speedup carbonate deposit?
In response to comment #1.
It took on the order of 100 kyr for the oxygen and carbon isotopic signals in the ocean to return to their original steady-state values after the PETM. The response time of atmospheric pCO2 ought to be faster than the response time of carbon isotopes, but then it is unclear to me what would be keeping the temperature up, if not higher CO2. Gavin Schmidt pointed me to a source (Bains et al., Science, 1999) where he sees different recovery times between 13-C and 18-O. I think the data plotted in Zachos et al. (Science 2001) is clearer, and I don’t see any difference between the recovery times. Still some loose ends.
In response to comment #3.
You’re correct that natural processes take up half or more of the carbon we release each year. The ocean takes up some, with no biology involved, just chemistry, and forests apparently take up the rest. The other nearly-half, however, still accumulates.
What if you were spending money from your checking account twice as fast as you earned it? Can you calculate how long it will take to get out of debt based on the ratio between income and outflow? Not really; they are different questions.
We could cut emissions by half, and the CO2 would stop going up. That’s what IPCC stabilization scenario model results essentially showed, and it demonstrates how Kyoto was just a baby first step in what would have to be done to prevent CO2 from rising beyond some level. If this were done pCO2 in the atmosphere would sort of float at the present higher-than-natural level. We would still be adding carbon to the atmosphere / ocean system, and it would still theoretically take 10^5 years to get pCO2 back down to a natural interglacial value of 280 ppm.
In response to Comment #4
None of the IPCC projections thought that emissions would remain at 6 Gtons C / year. The business-as-usual predicts a release of 1600 Gton C by 2100. For the 300 Gton model run, 17% remained after 1 kyr, and for 1000 Gton, it was 19%. After 100,000 years, the atmospheric fraction was 7%. Your “reasonable people” are right, the bulk of the carbon goes away in centuries. But there is that long tail.
The way the terrestrial biosphere fits into this is that, to bring CO2 back down to its preanthropogenic value, the biosphere would have to take up an amount equal to what we have emitted. The terrestrial biosphere is about 500 Gtons C, with soils another 1500 Gton C or so. It is reasonable to hope that the terrestrial biosphere will take up some hundreds of Gton C, but the land carbon reservoir would have to double if we wanted it to take up thousands of Gton C. That’s asking a lot, especially on a heavily populated planet. There are several mechanisms that govern terrestrial uptake of carbon. One is CO2 fertilization, another is the longer growing season. On the other hand, soil respiration rates increase as temperatures rise. Tropical soils don’t have much carbon in them for this reason.
I don’t know of a myriad of processes that can neutralize CO2 in the ocean without CaCO3. Surface ocean pH values are going down, that’s a fact. The model neglects corals and shallow-water CaCO3 accumulation, so the time scale for neutralization could be faster. The final atmosphere / CaCO3 buffered ocean equilibrium value of 7% remaining in the atmosphere, however, is simple thermodynamics, unaffected by uncertainties in the kinetics of neutralization.
In response to comment #5
One hundred years makes perfect sense on a societal level. I don’t know anything about the legality of the 100 year time window for Kyoto negotiations, though.
Growing trees that then release their carbon 100 years from now might reduce the size of the transient atmospheric high level, without changing the long climate tail. It is interesting to note that some of the carbon sequestration proposals are not much better in this regard. Pumping CO2 into the deep ocean lowers the atmospheric transient but not the long tail. I’ve heard a leakage rate of 0.1% per year tossed around as acceptable for terrestrial carbon sequestration; pumping CO2 down into the ground. This would give the storage a 1000 year lifetime, again without changing the long tail.
In response to comment #10
The sink has gone up because the atmospheric pCO2 has gone up; it’s farther from the equilibrium value with respect to the ocean. The same models that get the ocean sink right today also predict that not all of the carbon will go into the ocean (if the researcher runs the model that long). The existence of a significant sink today does not affect the conclusion of the long tail, nor even does the observation that the sink today is growing.
In response to comment #7
The ocean model I used does carbon chemistry, with plankton and gas exchange and all that. It does a pretty good job of mimicing the distributions of lots of geochemical and physical tracers. I expect that it should do a pretty good job of mimicing the “Revelle factor” change as CO2 goes up.
The circulation field of that model is “frozen”, that is, it does not respond to changes in climate. It’s missing a potential positive feedback there, as the ocean might be stagnant for awhile, slowing CO2 invasion.
I drive the mean ocean temperature using changes in atmopsheric CO2 with a time lag of 1000 years. I’m using a deep ocean “climate sensitivity” of 3 degrees C for doubling CO2, from Stouffer’s recent work. If that’s wrong in one direction or the other, the temperature feedback would be wrong. As is, it increases the long time-during fraction by about 20%.
The rate of CaCO3 production by plankton is taken to be fixed in time, as is the rate of weathering on land and the rate of shallow-water deposition. These would probably be negative feedbacks, making the relaxation toward equilibrium somewhat faster than I got. This wouldn’t change the amount of the long-duration fraction.
I didn’t model terrestrial carbon at all, but rather lumped it together, including terrestrial uptake, into net emissions. There is more fossil fuel carbon available than the size of the terrestrial biosphere, including soil carbon, so I didn’t bother with it. Small potatoes.
Does anybody know where to find updates of global CO2 emissions?
The official source http://cdiac.esd.ornl.gov/trends/emis/tre_glob.htm stops in 2000…
The article states that the up-down exchange fluxes only differ by about 2 gt/yr. But this exchange is primarily from a bulk flow of water. Thus if the carbonate concentration in the upper ocean water increases, the downward movement of carbon will increase and the difference will increase. Do you have any historical data which would indicate how the fluxes have changed over time? I know that figure 4 in climate change 1994 showed that there was actually a net upward movement of CO2 (100 GtC/yr up and 91.6 down) from water movement. It was only the net 10 GtC/yr downward from marine biota which produced a net downward flow of carbon. And if the surface concentration of carbonate increased by exchange with the atmosphere there’s lots of room in the ocean depths to store it before it would cause much difference in the concentration of carbonate in upward moving water. So I don’t think it’s fair to assume that the 2 GtC/yr will not change. It most likely will increase in step with increased carbonate concentration in the upper ocean.
Re: #18
Thanks for your response, David. You said
In the Feely et. al. paper “Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans” (Science 2004 305: 362-366), the conclusion is that CaCO3 production is decreasing as surface seawater pCO2 (acidity) rises. Although, as you state, the feedback appears to be negative, there is also some question raised in the Feely paper of what the effects will be on overall phytoplankton primary production. I wonder what the effects will be on this production on various timescales and what will happen to deep ocean overturning and the so-called “biological pump” on long timescales.
By the way, there are two articles “The Climate Change Commitment” by Tom Wigley and “How Much More Global Warming and Sea Level Rise?” by Gerald Meehl et. al. in this week’s Science 18 March 2005. Here’s a summary from the magazine New Scientist Ocean heat store makes climate change inevitable. Both studies (which I have not seen) focus on warming in the pipeline and no doubt involve modelling of future ocean warming.
Is anyone accounting for greenhouse gas emissions from volcanism? Will volcanism increase as global climate continues to warm? That will influence how long global warming lasts. It took the climate about 50 million years to cool after the PETM all things considered… except human activity.
David Archer wrote:
“On the other hand, nuclear waste lasts for millions of years for some isotopes such as iodine 129. The public seems to find this information relevant, so the true longevity of anthropogenic climate change might be considered relevant to here-and-nov decisions as well.”
This throw-away comment reflects a mass-media conventional wisdom about radiation and associated health and safety issues. The flip side of a long half-life is that not many I-129 atoms are decaying right now. When I arrived at the Hanford Site 25 years ago, part of the folklore (expressed as a joke) was that the carcinogenic dose of I-129 was 28 pounds of it in the thyroid. This folklore is supported by research reported in grey literature and the peer-reviewed literature by Joe Soldat and his colleagues at the Pacific Northwest National Laboratory in the 60s and 70s. Basically the issues are that I-129 has a relatively “soft” beta decay (75 kev), a relative short biological half-life in the body, and an extremely low specific activity (that is, number of decay events per second per unit mass).
The specific activity of I-129 is so low that it is relatively difficult to measure it in the environment (or was — 15-20 years ago the method of choice was to zap the sample with neutrons so as to transmute the I-129 into something that could be more easily measured.). I-129 does have significant value as a tracer of movement of fission products through the environment and the biosphere, since any of it you find was created in the last 60 years and the quantity is essentially unchanged in that time.
Jim Dukelow
The global anthropogenic emission data set ends in 2000.
http://cdiac.esd.ornl.gov/trends/emis/tre_glob.htm
Does anybody now about recent updates?
Re.# 9
Pertinent questions – perceptive comments. Surely global emissions can only rise as the ‘slash and burn’ economic model develops in the east. There is a lot of quibbling amongst countries as to who can/should/does emit what – the bottom line is global.
I live in Denmark – a country with a carefully manicured image of environmental friendliness, accepted here and abroad. The country is, however, in almost complete denial on the subject of global warming – not only whether something should be done, but whether humans are involved at all. This scepticism permeates not only the population, but the media and institutions too. When a report on the subject does slip through it’s usually with a sceptical tone….
In response to comment #12
If we raise the CO2 concentration in the atmosphere to Cretaceous levels and hold it there for 10,000 years or so, the CaCO3 cycle in the ocean will restore the carbonate ion concentration back toward CaCO3 saturation. So the CaCO3-secreting critters that comprise the white cliffs of Dover were not living in an acid world at all. The acidity comes in the transient.
Tangentially, Caldeira (Nature, 2003) points out that the change in atmospheric CO2 now and in the near future is much faster than natural changes from the ice core record. The CaCO3 cycle was able to keep up with the 10-kyr CO2 rise as the ice sheets melted, but it won’t be able to keep up with the 100-yr anthropogenic CO2 rise. So the ocean will be much more acidified than occured naturally within the ice core record (now 800 kyr).
In response to comment #20
The 2 Gton / yr net CO2 invasion into the ocean is driven by the rising atmospheric concentration, not by water flow or heat fluxes or anything like that. The preanthropogenic atmospheric CO2 concentration was stable, indicating that at that time the fluxes into and out of the ocean balanced. There were some locations, first among them the equatorial Pacific ocean, where CO2 outgassed, and other locations where CO2 invaded the oceans. The net flux, driven by the rising atmospheric CO2 concentration, may rise for a while, but it could also fall as the buffer capacity of the seawater is exhausted (see comment #7, for example.
In response to comment #22
Volcanoes are generally thought to be imperturbable outside drivers of climate; I don’t know of any possible feedback that could make volcanic emissions respond to changing climate.
Volcanoes release CO2, both from metamorphic cooking of CaCO3 back into igneous rocks, and from juvenile carbon (mostly released at mid-ocean ridges). In the natural steady state, this release is thought to be balanced by weathering of igneous rocks (essentially the reaction looks like CaO + CO2 => CaCO3). Because the weathering of igneous rocks could very easily be climate-responsive (there is no weathering in the Antarctic dry valleys, for example), these processes combine to stabilize CO2 concentration and climate on time scales of hundreds of thousands of years. It is called the Silicate weathering thermostat, and has been extensively modeled by Berner and colleagues. Berner has a new book about this topic, pricey but elegant.
(not for publication)
If you would have an acronym dictionary on the site, it would be great for your simply-interested/non-technical readers. Like me.
Thanks.
[Response: See our glossary. -mike]
Skiping ahead, what if emissions are brought down to whatever it takes to keep it steady.
What is the limit of global release to keep the balance at 280 ppm?
Is it even possible with todays usage of fossile fuel versus energy needed?
re: #19 and #24
I received from Tom Boden a global emission update through 2002
graphs here:
http://hanserren.cwhoutwijk.nl/co2/sink.htm
In response to comment #30
We’ve left 280 ppm far behind. 280 ppm is the preanthropogenic concentration, from 1750 or earlier. The CO2 concentration today is 365 or something like that, going up at about 1.5 ppm per year. In my model, that is to say neglecting surprises but just considering the atmosphere / ocean / CaCO3 system, if we stopped releasing CO2 today and closed the terrestrial biosphere to either releases or uptake of carbon, just closed the system, CO2 would relax down to some value higher than today. We’ve released about 300 Gton C so far (unless Ruddiman’s right and we started 8000 years ago). From a table in reference #7, 16.8% of a 300 Gton release will still be in the air in 1000 years. A convenient conversion factor is 1 ppm about equals 2 Gt C in CO2 in the atmosphere. So 17% of 300 Gt C is about 25 ppm higher than preanthropogenic, or about 305 ppm in the year 3000.
Surprises that could change this include going into a glacial climate state, which had the mysterious ability to draw down CO2 in the past, or uptake by the terrestrial biosphere. The total biosphere including soil carbon is about 2000 Gt C, so it’s not absurd to imagine growing it to 2300 Gt C. But it would have to take up the entire 300 we emitted, is the point.
David,
I don’t think it is justified, given the noisy data to assume a mechanism other than a first order Fick’s diffusion.
Using a simple first order Fick’s law diffusion fit on the emissions and concentrations
1958-2002 yieds – with an equilibrium value of 280 ppm – a diffusion constant of 0.98135.
The 2002 level is 373 ppm (excess of 93 ppm above 280 ppm)
1) Stopping emissions completely in 2002 gives values of 320 ppm in 2050 and 297 in 2100, which is a decimation of already 82% in 98 years.
2) reducing emissions with 2% per annum gives a peak of 391 ppm in 2028 and a reduction to 348 ppm in 2100
3) holding emissions at 2002 level gives a growth to 428 ppm in 2050 and 453 in 2100
4) increasing emissions annually with 1% gives 463 ppm in 2050 and 596 ppm in 2100
The emission trend of the last 55 years shows an unintentional decreasing emission growth. No SRES scenario assumes a continuation of the current decreasing trend.
see:
http://hanserren.cwhoutwijk.nl/co2/co2fick.xls
re comment 27: Your point is that the amount the oceans can absorb from the atmosphere is determined /limited by the condition of the surface waters. But that implies that the amount of carbon being carried from the surface to deeper waters is fixed, or nearly so. The point of my previous message was to say that it isn’t.
Further, if in the past we had roughly 90 GtC moving in both directions between ocean and atmosphere, it would be absurd to claim that this value was somehow fixed and that changes on the order of a few percent in either direction would totally change things. Most negative feedbacks require a fairly substantial disequilibrium before they are very noticable. This is why I ask if there is data about the flows between surface and deep waters in the past. It’s possible there’s an upper limit to flows based on pH, but it’s more likely that instead we’ve been seeing the lower limit on flows in the recent past as the biosphere draws down the CO2 level as far as it can and still operate. Increased atmospheric levels of CO2 up to several doublings will likely just increasingly rev up the biosphere. Since it’s unlikely we’ll do much more than double the prehistoric CO2 level before using up cheap fossil fuel, I’d be very surprised if anything untoward occurs.
Is there a delay between the release of a tonne of CO2 in the atmosphere and the moment that this tonne starts to impact the earth’s climate?
I have heard theories that what we are seeing today is the result of C02 released 100-120 years ago, and that today’s releases will start to have their effect on our climate in 2120.
I would like to know if this theory has a scientific basis.
Thank you,
In response to comment #35
The CO2 appears in the atmosphere immediately, of course, at which point it begins to wind its way into the oceans and the biosphere. Wigley made a controversial but I think correct point a few years back that if the real concern of Kyoto and IPCC is the really severe climate change at the end of the century, the cutting emissions now will have less effect on that than cutting emissions closer to that time. Emissions now will have had some time to soak into the ocean.
There is however a time lag for the warming, which is what you’re probably thinking of. It takes a long time, decades, to warm the interior of the ocean. This means it takes a long time to ramp up to the full steady-state climate impact of a slug of CO2.
Regarding post 32
Posting 87 includes “The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2 to nearly 500 ppm.” So 500 billion tons would be enough to raise from 280ppm to nearly 500 ppm which looks more like 1 ppm equates to 2.5 billion tons. On this basis a 100ppm increase (from 280 to 379 (not 365)) would be more like 250 Gton of carbon instead of 300 Gton.
I think it would be sensible to sort out these numbers.
In JGR Atmospheres, February 15, 2005 there is a paper by a Stanford prof. which claims “lifetime of CO2 ranges from 30-95 yr although a more likely upper limit may be 50 or 60 yr.” See section 1 in: http://www.stanford.edu/group/efmh/fossil/Climresp_updateJGR.pdf
I have no expertise in this area but I would like to understand why there is such a difference is estimates of CO2 lifetime. Any thoughts ?
In response to comment 34
Several sources list the mass of the atmosphere as 5.3E18 kg. Assuming an average molecular weight of 28.8 (80% N2, 20% O2) gives us 2.21 Gton C per ppm of CO2. Sabine et al (Science 305: 367, 2004) seem to be using 2.1 Gton C per ppm (consistent with an atmospheric mass of 5.0E18 kg). Let’s assume that Sabine is right and I’m wrong.
Sabine has budgets of emissions and current perturbed inventories as well. They have emissions to 1994 of 244 Gton C. CDIAC (link in comment #24) says 283 Gton C to the year 2000. The ocean inbentory (the whole point of the paper) was 118 Gton C. Sabine figures a net release from the terrestrial biosphere of 40 Gton C between 1800 and 1994. So if we put together a combined net source, it would be close to 320 Gton C, which could have raised pCO2 by 150 ppm, to about 430 ppm.
500 Gtons of emissions seems out of the ballpark; could it have been Gton of CO2, rather than Gton of C? Mass units for chemistry are endlessly confusing.
In response to comment #36
Wow, this was interesting, thanks! The problems with Jacobson’s calculation are (1) the assumption that the atmospheric pCO2 is relaxing toward preanthropogenic in Jacobson’s equation 1, and (2) the wide range of uptake time scales for different parts of the ocean, which interact with the accelerated rise in atmospheric pCO2. I wrote up a simple model in excel which puts CO2 into the atmosphere and takes it into the ocean according to various reservoir sizes and ventilation times. The estimated uptake timescales are within the range he reports for his data-driven calculation, 50 years or so, even though the mean uptake time of the ocean reservoirs in that model, weighted by their sizes, is 600 years. I have emailed my spreadsheet, and a mention of realclimate.org, to Dr. Jacobson.
It is obvious that we’ll not avoid the anthropogenic greenhouse effect.
What are you thinking about a fight against this effect by , for example ,the injection in the higher tropsphere of very tiny (1 to 5 microns)aerosols?
These aerosols must be chosen transparent to the IR and opaque to the visible radiation.
In another way what are you thinking about the artificial injection of micronised CaCO3 or MgCO3 in the oceans to neutralize atmospheric CO2?
thank you to apologize my bad english.
In response to comment #33
The ocean uptake physics are governed by ocean circulation, which we know a lot about by measuring the carbon-14 distribution in the ocean. This is not noisy data. The constants governing the carbon chemistry in the ocean are known to many significant digits, from nice well-behaved laboratory data. The thermodynamic formalism of how one does carbon chemistry calculations are almost fundamental laws of science. The rate of gas exchange, I’ll grant you, is a noisy function of wind speed, but the long-term uptake is not terribly sensitive to gas exchange; it’s circulation that matters primarily in the long run. .
The equilibrium pCO2 is not 280 ppm. This is an assumption you are putting into your calculation, not a conclusion you are taking out.
In response to comment #41
With regards the idea of decreasing earth’s temperature by injecting aerosols in the stratosphere: I am not an expert on this, but I haven’t heard any reasons why it wouldn’t work. The climate cooled down significantly, for several years, after Pinatubo.
With regards neutralization with CaCO3 or MgCO3: It might be easier to do the chemical reaction in high-CO2 flue gas from power plants, rather than try to make it work at seawater temperature and pH. I would think that neutralized CO2, in the form of bicarbonate at seawater pH, would be much less harmful to ocean biota than would unneutralized CO2, either directly injected or by passively letting it invade the surface ocean from the atmosphere.
Re: # 42
Dave,
Considering the vostok Ice core the equilibruim CO2 level is a fairly linear function of temperature (9.8 ppm/K)
I agree that the pre-industrial equilibrium of 280 ppm should now be 287 ppm, but that increase is still small compared to the anthropogenic signal of 93 ppm.
Secondly the complete atmospheric reservoir of 730 GtC is negligable compared to the oceanic reservoir of 38000 GtC using first order principles.
In response to #43
thanks for your response
I’m agree with you for the neutralization of high-CO2 flue gas from power plants.
I had the idea to put micronized CaCO3 on the surface of the oceans primarily to increase the global albedo of the Earth .
It was the same idea that the aerosols.
I thought that CaCO3 was a good candidate for this purpose .
It was a natural and very abundant element ,it could give a very little “milky” aspect to the oceans ,maybe sufficiently to increase the albedo.The increasing of the pH of the ocean was a little plus.
But it is an evidence that it should’nt be a low-cost operation.
WRT # 43 and 41: Good luck finding an aerosol that does not absorb in the IR:). Almost by definition an oxymoron because of the relationship between vibrational motions and IR absorptions.
re #43
David,
I am aware of the temperature sensitivity of the CO2 equilibrium of the atmospheric CO2 level: the vostok ice core record yields a sensitivity of 9.8 ppm/K. therefore the current equilibrium should be 288 ppm (0.8 K higher as in preindustrial times).
Secondly the total atmospheric CO2 reservoir is negligable compared to the oceanic reservoir (730 GtC vs 38000 GtC) in first order diffusion effects.
How long will global warming last?
My perdiction: 500 years
My Opinion: The icecap over Greenland will melt over the next 500 years. At that point, the Atlantic Ocean current will reverse and initiating a cooling period (http://www.iscanadaready.com).
Re #48 & since this question/topic invited rampant speculation by its choice of a title:
My prediction is closer to 10-20,000 years.
Since this is when the orbital fluctuations that supposedly caused the last ice ages willl reverse and we will START to get colder because there will be less energy coming into the earth from the sun.
My problem with the 500 year scenario is that CO2 induced warming increases the amount of energy staying on earth, thus melting (some of?)the Greenland ice. BUT Reversing the Atlantic ocean current due to fresh water ice melt, is a local phenomenon, not global AND it does little to reduce the slow steady heat/energy buildup globally – so warming will continue.
As for the CO2 increase, if the price of oil keeps going up at its current rate, $20/barrel in the 90s, $58 today, some broker estimated $105 in the near future, THEN solar energy becomes very affordable very quickly. The “dreaded” oil companies will rush to make a solar profit. CO2 goes down. CO2 induced warming goes down, BUT the Milankovitch orbital mechanics still dictate that in 10,000 years we will be closer to the sun, & probably warmer.
Speculating is such fun. You can make a case for just about anything!!
Time to close this topic guys!